Cylinder-piston arrangements which do not use piston rods have a particular advantage: they require only little longitudinal space, and, additionally, have the advantage that problems associated with the piston rod extending from the cylinder are avoided, for example damage to the surface of the piston rod, bending thereof, or the like. The force applied to the piston by a pressure fluid, which may be hydraulic or gaseous, is directly transferred by the piston on an externally extending force transmission element, guided in a longitudinal slit of the cylinder tube. The slit, through which the force transmission element extends, must be sealed. Sealing of the pressure spaces at both sides of the piston is effected in radial direction with respect to the cylinder tube by sealing rings or sealing sleeves formed on the piston; the longitudinal slit of the cylinder tube is sealed by a sealing strip which, in the unpressurized region, is guided between the piston seals and the force transmitting element.
Sealing of the longitudinal slit presents problems. It has been proposed--see German Pat. No. DE-PS 843 482--to provide a sealing strip of essentially rectangular cross section, located in an interior longitudinal groove which, also, forms lateral guide surfaces for the sealing strip. The pressure within the cylinder, when pressurized, presses the sealing strip against the engagement surface. Practical experience has shown, however, that it is difficult to prevent the sealing strip, when unpressurized, to hang-through into the pressure chamber, since, otherwise, upon admission of pressure fluid, an initial leakage thereof will result. Hang-through is practically unavoidable.
Another cylinder of this type is described in German Pat. No. DE-PS 846 493, in which a sealing strip having essentially U-shaped cross section is used. The flange of the sealing strip is fitted in corresponding recesses within the interior wall of the cylinder tube, in order to improve sealing of the pressure spaces and simultaneously to improve sealing of the pressure spaces and simultaneously to improve the resistance of the cylinder tube with respect to expansion of its diameter upon pressurization. This, however, results in an extremely expensive and complex structure, difficult to make, entirely apart from the fact that the piston must have a particular and special construction in order to permit guidance of the sealing strip with its flanges in the respective recesses of the cylinder.
It has been proposed to simplify such an arrangement--see German Pat. No. DE-PS 21 62 572--to make the cylinder of a non-magnetic material which, at least in one of the wall regions parallel to the longitudinal slit, has a magnetic body, and to associate therewith a sealing strip made of magnetizable material. in order to be attracted by the magnet of the magnet body. Such a sealing strip is metallic; micro-leakage is practically unavoidable, so that this type of cylinder seal can hardly be used with liquids; such a cylinder, practically, thus can be used only with compressed air. In practice, it has been found that the magnet elements used to hold the sealing strip also attract extraneous ferrous articles, such as iron chips and the like, which interferes with proper positioning of the sealing strip and, generally, may lead to substantial operating interruptions. The sealing strip which is used is a very thin metal tape or web which is not laterally guided by the magnetic elements. Under some operating conditions, the metal tape can be laterally shifted which may lead to explosive dislocation of the strip, upon pressurization, and thus presents a substantial hazard in operation.
It has been proposed to utilize a metallic covering strip located at the outside of the cylindrical tube at both sides of the force transmitting element, covering the longitudinal slit. This cover strip is used to cover the longitudinal slit towards the outside, and thus prevent penetration of dirt and the like therein. Such a metallic cover strip can be held by magnetic elements. Magnetic holding of strips within, as well as without, the cylinder tube is expensive, however, since special steel strips must made, accurately ground to size to match the cylinder and the surfaces to be covered or sealed.